A variety of wireless communications systems are used to provide voice and data transmission to and from subscriber telephonic units within a prescribed geographic area. Analog cellular systems such as those designated as AMPS, ETACS, MMT-450 and MMT-900 have been deployed throughout the world. Subsequently, digital cellular systems have become common, and include the so-called personal communications system (PCS) including those denoted as IS-95m OCS-1900, PACS, DCS-1800, DECT, PHS and other names. PCS systems generally utilize a radio frequency band of about 2 gigahertz (Ghz) for both voice and data transmission.
In a conventional terrestrial wireless communication system, wireless mobile terminals, i.e. cellular phones may communicate with a plurality of cells served by base stations and a mobile telephone switching office. Communication is enabled between mobile telephones and between a mobile telephone and a land line terminal, e.g. conventional telephone or computer.
Celestial wireless communications systems are known, and comprise typically a plurality of earth satellites that serve as transponders or relays for radio communications between earth stations and wireless satellite mobile terminals. Such cellular systems may utilize a single antenna beam covering the entire area served by the system, or alternatively, the system may produce multiple beams (with minimal overlap), each beam serving a distinct geographic coverage area within the service region. Thus, a satellite and beam coverage serve the same purpose as a base station and a cell of a non-satellite cellular communications system.
Various systems for locating one's earth position are in place including the well-known Global Positioning Satellite System (GPS). In this application, electronic position locating systems will be referred to generically as physical location systems (PLS) of which GPS is one version.
Various methods and apparatus for integrating a physical location system (PLS) such as a global position satellite system (GPS) with a satellite communications system are described in the patent literature, as noted below.
In U.S. Pat. No. 5,448,623 of Wiedeman et al., a satellite communications system is disclosed having a network of gateways which interface with a terrestrial communications system.
U.S. Pat. No. 5,946,618 of Agre et al., describes a mobile telephone system employing a fleet of communications satellites and a set of gateway ground stations arranged around the globe for handling communications to and from mobile subscriber units. The location of a mobile unit is determined using a GPS satellite system, and the mobile unit transmits the location to a gateway. The disclosure presents methods for selecting which service provider will handle a call made by a mobile unit, and ways for directing emergency calls to the proper authorities. In addition, methods for handling a mobile unit call to a nearby phone which is far distant from the calling unit's home area are disclosed, wherein the call is handled as a local call irrespective of whether a long distance area code or trans-country prefix is dialed.
U.S. Pat. No. 5,995,041 of Bradley et al. discloses a satellite phone for direct communication with a communications satellite system. The phone may be stationary or mobile, and generates a highly directed antenna beam which is continuously controlled regardless of the position of the phone, using GPS determined coordinates of both the phone and a communications satellite.
In U.S. Pat. No. 6,028,514 of Lemelson et al., a system is described for monitoring a person's location. A few standardized warning messages may be transmitted to the person, and a few emergency messages may be transmitted from the person's unit to a command center. The person's location is calculated from GPS signals, and the communications are transmitted via a terrestrial cellular system or via communication satellites.
U.S. Pat. No. 6,041,222 of Horton et al. describes a wireless mobile terminal having a communication transceiver and a GPS transceiver. The two transceivers share a common frequency reference signal which is used for signal demodulation and processing. The reference indicates utility in an advanced “911” system whereby an emergency call by e.g. an accident victim would include the GPS location of the victim. It is also indicated that radio communications from a communications satellite could be highly directed (in “pencil beams”) if the mobile terminal's location is accurately known. This would save considerable transmission energy.
In U.S. Pat. No. 6,064,336 of Krasner, a GPS receiver having a communications link is described. A precision carrier frequency is used to calibrate the local oscillator such that its output is modified by a reference signal generated from the precision carrier frequency signal. More particularly, satellite almanac data are transmitted to a remote GPS receiver unit from a base station. The remote GPS receiver unit uses the almanac data to determine approximate Doppler data for the satellite in the “view” of the receiver unit.
As described in U.S. Pat. No. 6,061,561 of Alanara et al., a mobile station within the geographic limits of a “cell” receives a signal from the cellular base station indicating the location of the base station. The location data are stored in the mobile station and transmitted in calls made therefrom.
In U.S. Pat. No. 6,002,363 of Krasner, a cellular mobile unit is disclosed which has a GPS receiver for determination of the unit's location. The circuitry for GPS receiving/calculating operations and cellular communications operations are shared, requiring that both operations cannot be used simultaneously.
U.S. Pat. No. 5,890,092 of Kato et al. describes a navigation system for vehicles wherein direction of movement (heading) is calculated using a geomagnetic sensor, steering sensor, gyro, or the like. The heading data is combined with GPS position data to determine the route and velocity of a vehicle.
As disclosed in U.S. Pat. No. 6,044,257 of Boling et al., a “panic button” cell phone may be activated to receive GPS signals and calculate its position. The phone may be used to call to an emergency response center (ERC) which is given the phone's position. In addition to a sound alarm, a low-range radio may be used to activate a household alarm system.
A cellular communications device is described in U.S. Pat. No. 6,067,082 of Enmei in which its position is calculated from GPS signals, and transmitted in communications (voice, facsimile and data) made from the device.
In the current state of the art, each cellular telephone unit has a unique identification code. Radio signals (initiated by other units) which present that code will initiate a response from the called unit, as long as the called unit is “switched ON” and is within radio range to receive the radio signals. Cellular telephone communication with ships at sea is non-existent unless the ship is in port or near land having a nearby cellular tower.
Currently, radio communications at sea require a person to maintain a radio ON and listen to various parties communicating on the same channel. In proper use, the radio must be kept ON and audible. Because it is tiresome to continuously listen to conversation which is not specific to the listener's vessel, radios are generally ignored, and often are turned OFF. Thus, a call from another vessel is unsuccessful. Often, distress (SOS) transmissions from nearby ships or boats are missed, and the vessel passes the troubled ship without any knowledge of the emergency situation. Plans are underway to abandon the present maritime communications system in the near future because of these and other inherent problems. In a proposed system, a ship in trouble may dial a sighted vessel (if its “telephone” number is known), and an emergency button may be activated to radio-transmit the GPS position of the calling “phone” and a MayDay Message to the sighted vessel. Only short range “line-of-sight”VHF radio is in view, and the phone number of the rescue vessel must be known. Thus, a satisfactory and comprehensive communications system for providing enhanced safety considerations has not yet been announced.